In one conventional solution, a Licensed Assisted Access (LAA) E-UTRAN NodeB or evolved NodeB (eNB) transmits data or control information immediately after it finishes the preamble transmission when measuring the channel as clear. A transport block is transmitted over a fractional subframe. This allows the LAA eNB to start the downlink transmission as soon as possible. This may result in mismatch between Modulation and Coding Scheme (MCS) selection and Transport Block Size (TBS) selection. Long Term Evolution (LTE) system supports adaptive MCS, which enables the transmitter to choose the proper MCS index based on Signal to Interference and Noise Ratio (SINR) condition. Basically different MCS corresponds to different SINR condition in order to reach some design target, e.g. optimized transmission efficiency or fulfilling a given Block Error Rate (BLER) target. The mapping between MfiCS index and TBS index is based on a baseline assumption, i.e., a predefined number of OFDM symbols used for Physical Downlink Shared Channel (PDSCH). It would cause significant error for mapping between TBS and MCS if the number of Orthogonal Frequency Division Multiplexing (OFDM) symbols used for the PDSCH is dramatically different with the baseline assumption.
In another conventional solution, a transport block is transmitted over a fractional subframe and the subsequent subframe. As the number of OFDM symbols used for a transport block is different with the baseline assumption, TBS scaling is applied where the actually used TBS is scaled from the indicated TBS in the DownLink (DL) grant to match the TBS and MCS. In addition, this may result in poor support of User Equipment (UE)-specific reference signal port 7 and port 8 simultaneously, or poor support of UE-specific reference signal port 9 and port 10 simultaneously, or poor support of UE-specific reference signal port 7/8/9/10 together with UE-specific reference signal port 11/12/13/14.
As specified in LTE, UE-specific reference signal port 7 and UE-specific reference signal port 8 are code multiplexed using a length-2 code, while UE-specific reference signal port 9 and UE-specific reference signal port 10 are code multiplexed using a length-2 code. In addition, UE-specific reference signal port 7/8 and UE-specific reference signal port 11/12 are multiplexed using a length-4 code, while UE-specific reference signal port 9/10 and UE-specific reference signal port 13/14 are multiplexed using a length-4 code. In case a fractional subframe contains an odd number of OFDM symbols carrying UE-specific reference signals where the preamble ends at OFDM symbol 6 and in the fractional subframe there are only 3 OFDM symbols carrying UE-specific reference signals, it would be difficult or even impossible for the receiver to distinguish the code-multiplexed reference signals which need 2 or 4 code multiplexed signals to decode, e.g., UE-specific reference port 7 with UE-specific reference port 8, and UE-specific reference port 7/8 with UE-specific reference port 11/12.
The disadvantages of these conventional solutions at least include: mismatch between MCS selection and TBS selection or additional complexity caused by TBS scaling; poor or even no support of UE-specific reference signals port 7-14, especially in the case the fractional subframe only contains an odd number of OFDM symbols carrying UE-specific reference signal.